chmAddDataset-method: Add an auxiliary dataset to a NGCHM.

Description

Add an auxiliary dataset to a Next Generation Clustered Heat Map (NGCHM) and return the extended CHM.
The auxiliary dataset will be stored with the NGCHM and be available in whole or in part from the
same server, for use, for example, in custom Javascript functions.
Do not confuse this function with the one for adding an active data layer to the heatmap itself. For
that, please refer to the function chmAddLayer.